The commercial use of untreated carbon as fuel results in excessive pollution and maintenance difficulties in industrial plants due to presence of contaminants in the carbon. Also when a carbon structure, such as graphite, is used for anodes or cathodes in electric industrial applications or where graphite is used for lubricants, contaminates can cause pollution, poor conductivity and wear mechanical parts. The de-contamination of carbon has been a major problem which has resulted in the range of uses of coal as a basic fuel being severely restricted because of environmental requirements. In addition, installation costs of industrial plants which use raw coal for fuel are high because of the need for pollution control equipment to scrub exhaust gases. There are also high maintenance costs associated with cleaning of such plants as contaminates form coatings on surfaces exposed to coal combustion and exhaust gases. For a carbon structure of graphite, contaminates therein reduces its economic value as well as resulting in environmental pollution in electric industrial applications. Where graphite is used as a lubricant contaminates therein also reduce its economic value and effectiveness as a lubricant.
It has been found that contaminants in carbon can at least in part be removed by using hydrofluoric acid. The problem encountered in using hydrofluoric acid by itself to react with siliceous and other inorganic contaminants in carbon is that hydrofluoric acid is not selective and will react with almost all of the contaminates. The disadvantage of this non-selectivity is that Ca and Mg contaminates are converted to insoluble CaF.sub.2 and MgF.sub.2 which cannot be easily removed. The rate and extent of reaction is directly related to the temperature and strength of the acid.
The amount of HF necessary to totally react with metal oxides in the carbon is relatively small in comparison to the volume of carbon, so that this volume of acid alone would be insufficient to thoroughly wet the carbon. If a sufficient volume of H.sub.2 O is mixed with this acid to wet the carbon, the acidity level is lowered and this affects the reaction time, the degree of purification of the carbon, the ability of the acid solution to solubilize and carry the metal fluorides from the carbon and also creates a large waste liquid treatment requirement.
Once the carbon is wet with acid, it leaves the leach circuit as a slurry and goes to a physical solids-liquids separation process which is typically mechanically arranged through a centrifuge or vacuum filter. After the solids-liquids separation step, the carbon retains on its surface about 15% by weight of leach liquor. The 15% by weight is of the weight of the carbon structure plus the retained leach liquor. Since acid in the retained leach liquor can be lost to the process, it is necessary to ensure that the leach liquor used contains the least expensive HF component by weight in order to ensure that the process is economically feasible.
The metal oxides within the carbon are converted into fluorides and in most cases are taken into a solution. However, some metal fluorides are not soluble in hydrofluoric acid such as calcium fluoride. This creates a major problem in that being insoluble, they tend to block the pores of carbon and thereby restrict the time and the extent of the chemical reactions on other metal oxides which may be locked behind them in the carbon.
As the residual liquor remaining in the carbon after leaching is typically around 15%, much of the solution will be fluorine, attached to silica or alumina. The economic recovery of fluorides from these metals is costly.
The problem in reacting hydrofluoric acid with metal oxides is the high cost of fluorine as the chemical reaction: Al.sub.2 O.sub.3 +6HF.fwdarw.2AlF.sub.3 +3H.sub.2 O consumes a substantial amount of hydrofluoric acid relative to the aluminium oxide.
As a further example of this, the main loss occurs in the reaction: EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O
However, in the presence of aqueous solutions, the SiF.sub.4 will hydrolyze into H.sub.2 SiF.sub.6 and so give off SiO.sub.2 which will react with further amounts of HF so we now have a further loss of fluorides to the solution and therefore a higher cost in raw materials.